To keep global warming in check, the world will have to invest an average of around $3 trillion a year over the next three decades in transforming its energy supply systems, a new United Nations climate science report says. It won't be cheap, but it's also a change that's already underway.

Much of that investment is money that would be spent on energy systems anyway. Instead of continuing to invest it in fossil fuel-based energy that worsens global warming and can harm human health, the report provides a pathway for shifting those investments to clean energy.

The landmark report, released Oct. 8 by the Intergovernmental Panel on Climate Change (IPCC), sums up years of research into the risks to people and ecosystems if global temperatures rise 1.5 degrees Celsius above pre-industrial times, and it looks at how to stop that from happening. The planet has already warmed about 1°C, and it's gaining about 0.2°C every decade, the report says.

Keeping warming under 1.5°C will require a near complete shift from today's dependence on fossil fuels to a world powered almost entirely by clean energy, the IPCC says.

That transformation will require a global investment in clean energy and infrastructure of $1.6 trillion to $3.8 trillion a year (in 2010 U.S. dollars), with an average of about $3 trillion to $3.5 trillion a year from 2016 to 2050, the IPCC says. That compares to an estimated $2.4 trillion a year that would otherwise be invested in energy systems.